1. Field of the Invention
The present invention relates to methods and apparatus for measuring the shear stress required to remove drilling fluid deposits from the walls of well bores and for determining erodability factors for drilling fluids.
2. Description of the Prior Art
In the drilling of an oil and/or gas well, a rotary drill bit connected to a string of drill pipe is most commonly used. The drill pipe and drill bit are rotated, and a weighted gelled drilling fluid, e.g., an aqueous clay containing fluid having particulate weighting material suspended therein, is circulated through the well bore to lift cuttings produced by the drill bit to the surface and to maintain hydrostatic pressure in the well bore whereby pressurized fluids contained in penetrated subterranean formations are prevented from entering the well bore. The circulation of the drilling fluid is accomplished by pumping the drilling fluid downwardly through the drill pipe, through ports in the drill bit and then upwardly in the annulus between the drill pipe and the walls of the well bore.
When the drilling of the well bore is completed, the circulation of the drilling fluid is stopped while the drill pipe and drill bit are withdrawn, the well is logged and pipe, e.g., casing, is run into the well bore. During this shutdown period, significant quantities of filter cake and partially dehydrated gelled drilling fluid are often deposited on the walls of the well bore as a result of the drilling fluid remaining static in the well bore and the occurrence of fluid loss from the drilling fluid into permeable subterranean formations penetrated by the well bore. The filter cake is principally comprised of particulate weighting material and other solids, and the partially dehydrated gelled drilling fluid is formed from drilling fluid adjacent the walls of the well bore which develops gel strength in the absence of shear and loses a portion of its water as a result of the fluid loss. Also, the remaining drilling fluid in both the pipe and annulus develops gel strength in the absence of shear during the drilling fluid circulation shutdown.
After pipe is run into the well bore, primary cementing operations are performed therein. That is, the pipe is cemented in the well bore by placing a cement slurry in the annulus between the pipe and the walls of the well bore. The cement slurry sets into a hard impermeable mass whereby the pipe is bonded to the walls of the well bore and the annulus is sealed. When the cement slurry is run into the annulus, drilling fluid is displaced from the well bore thereby.
In order for a primary cementing operation to be successful, all of the gelled drilling fluid and at least major portions of the partially dehydrated gel drilling fluid and filter cake deposited on the walls of the well bore must be removed. If too much of the drilling fluid and filter cake deposits remain on the walls of the well bore, the cement will not properly bond thereto and highly undesirable fluid leakage into and through the well bore will result.
Heretofore, attempts have been made to remove the drilling fluid deposits in the well bore after the above described drilling fluid circulation shutdown period by circulating the drilling fluid through the well bore for a period of time prior to commencing primary cementing. That is, the drilling fluid is continuously pumped downwardly through the pipe to be cemented in the well bore and upwardly through the annulus between the pipe and the walls of the well bore for a period of time during which it has heretofore been hoped that major portions of the partially dehydrated gelled drilling fluid and filter cake are eroded and removed from the walls of the well bore. In attempts to determine if such circulation results in the erosion and removal of the drilling fluid deposits prior to displacing the drilling fluid with a water spacer followed by a cement slurry, marker fluids or materials have heretofore been combined with the circulating drilling fluid at the surface. The time required for the marker to flow through the well bore and reappear on the surface has been determined and such time has been multiplied by the pumping rate of the drilling fluid to estimate the circulating drilling fluid volume. The estimated circulating drilling fluid volume has then been compared with the calculated volume in the well bore available for containing drilling fluid to determine if major portions of the drilling fluid still remain on the walls of the well bore. This technique and other similar techniques for determining the circulating drilling fluid volume have not provided reliable information concerning whether drilling fluid deposits have been removed, and as a result, less than desired primary cementing results have often been obtained.
By the present invention, methods and apparatus are provided for measuring the minimum shear stress at the walls of a well bore required to erode drilling fluid deposits formed thereon prior to when the drilling fluid is recirculated after the above-described shutdown period. A knowledge of the minimum shear stress required allows the drilling fluid to be circulated at a proper rate to efficiently remove the drilling fluid deposits, or for special spacer fluid or other means to be employed to bring about such removal prior to placing a primary cementing slurry in the well bore.